This invention relates to a novel system for inerting combustible and potentially explosive fuel supplies in vehicles such as aircraft. More particularly, this invention provides a novel nitrogen fuel inerting system that will fill the ullage (i.e., the vaporous volume within a fuel tank above the liquid fuel) of an aircraft fuel tank or other fuel tank with an inert gas that will not support combustion. The invention concomitantly provides a capability of actively monitoring the oxygen content of vapor and gas in the ullage of a fuel tank to maintain the inert status of the over-fuel vapors and enable efficient dispensing of the inert gas.
Since 1959, there have been a number of explosions of the center wing tank on military and commercial aircraft reportedly resulting in deaths of as many as 550 persons. Those infrequent but continuing occurrences involving fuel tank explosions are believed to have possibly been caused by unknown sources of ignition, possibly initiated by conductive wires exposed to the explosive fuel vapor/air mixture in the tank. These accidents have added impetus to the search for an effective system for inerting flammable and potentially explosive vapors in fuel tanks, particularly of aircraft. Inerting systems using halogen-based gases have been known for use in military aircraft. But the use of halogen-based gases is not viable in commercial aircraft and in general aviation because of their effects resulting in ozone depletion. A non-halogen based system, moreover, would be advantageous in military applications as well because of the more environmentally friendly nature of an inerting system based on a non-halogenic gas.
A brief discussion of the history of the problem of center fuel tank explosions and of the growing interest in implementing an inerting system on commercial aircraft is found in Air Safety Week, Vol. 15 No. 16, Apr. 16, 2001, xe2x80x9cFatal Explosion Highlights Hazard of Flammable Vapors in Fuel Tanks.xe2x80x9d The discussion in that article pointed out that, in particular, the center wing tank of commercial aircraft can reach high temperatures when the aircraft required to queue up on the hot tarmac of an airport at summer temperatures, and that such conditions have been linked to some explosions. The issue of Air Safety Week noted above also discusses a number of nitrogen based inerting systems that were considered by the Aviation Rulemaking Advisory Committee at its meeting of Apr. 4, 2001. Included among those systems was a brief discussion of the system of the present invention.
Accordingly, this invention provides a nitrogen inerting system for potentially flammable and explosive fuel-vapor mixtures that exist in fuel tanks, particularly in aircraft fuel tanks. However, it will be apparent that the systems of this invention are readily adaptable to inerting fuel tanks in any type of vehicle and hence this invention may find utility in various types of military vehicles or in vehicles intended for operation in a hazardous environment or under other circumstances where there is sufficient danger of fuel tank explosion or sufficient desire to negate such a possibility so as to justify the installation of the system. It is also apparent that the system of this invention could be also employed to inert a non-fuel tank environment such as a cargo hold, or to provide a fire suppression system for selected areas of the aircraft or other vehicle, In such an instance, the nitrogen gas to inert an area might be released only after the potential of fire is detected elsewhere on the vehicle. Hence, although this invention will be explained herein in the context of an onboard nitrogen inerting system for an aircraft center wing tank, it will be understood that the system may be adapted to other fuel tanks or to multiple fuel tanks on aircraft and other vehicles, and can further be adapted to create an inerted environment or supplemental fire suppression system in a volume other than a fuel tank. Most likely this system will be used principally as a fuel tank inerting system, will be simultaneously adapted additionally to provide a fire suppression system in other regions of the aircraft, such as a cargo hold.
In particular, the instant invention provides a system that provides an inert blanket of a nitrogen rich gas atmosphere to a fuel tank or other environment so as to inert that environment from the prospect of explosion. In a further aspect of this invention there is provided a system with safe and effective monitoring of the oxygen concentration (preferably measured as oxygen partial pressure) in the atmosphere in a tank or other area to be inerted such that nitrogen can be metered efficiently from a liquid nitrogen source to maintain the atmosphere at a desirably low oxygen concentration so that the potential of explosion is minimized or eliminated. Temperature of the tank can also be monitored further to assess the potentially explosive character of the tank contents. There is further provided an effective probe which can effectively monitor the oxygen content of the atmosphere within a fuel tank, and which can accomplish that monitoring function accurately without introducing a danger that the probe itself can be a source of sparking that could initiate an explosion. In a special embodiment, the probe is a passive fiber optic probe that can be extended into the fuel tank or other environment to monitor oxygen content and provide a basis for controlling oxygen concentration. Because, in the particular embodiment of this invention, the output of the fiber optic probe indicative of oxygen concentration is temperature dependent, there is also provided herein a shroud or device to maintain the probe at a known uniform temperature in order to assure an accurate oxygen measurement can be obtained.
The system of the instant invention utilizes an on-board liquid nitrogen supply in the form of a dewar or similar liquid nitrogen-holding container which is suitably insulated to maintain the liquid nitrogen at reduced temperature for a long period. The container desirably is sized to provide a sufficient supply of nitrogen to blanket the fuel tank, for example the center wing tank or multiple fuel tanks of an aircraft, preferably several times over as will be explained below. The on board nitrogen supply is in valved fluid communication with the fuel tank or tanks or other environments to be inerted, most typically the aircraft""s center wing tank. A passive fiber optic probe that extends into the tank monitors the oxygen partial pressure within the tank. The output from the probe can be fed to a spectrometer or similar instrument that is adapted to translate the analog output of the probe to a digital signal which can be fed to a suitable controller, such as a microprocessor-based controller. The microprocessor can then control valves through which nitrogen flows from the onboard nitrogen source to feed nitrogen to the tank as needed to maintain the over fuel atmosphere in an inerted condition. The apparatus can control that flow and maintain an inert condition by monitoring a signal from the probe and using software adaptations that will be understood by those skilled in the art to enable the microprocessor to control the nitrogen content in the ullage of the tanks.
The accurate probes contemplated by this invention provide an output signaling oxygen partial pressure that is temperature dependent, and hence the invention also provides a receptacle or shroud to provide a stable temperature environment which can be used in the case of a system which is subject to the widely varying temperatures the will be encountered by a commercial aircraft. That is, the invention provides a heated receptacle or well to encase the probe extending into the fuel tank to maintain the probe at a sufficiently constant temperature to assure accurate oxygen readings by the probe. The fuel tank is supplied with a suitable relief valve that will expel the over-fuel gas-vapor mixture as nitrogen is metered from the on board supply to the tank. The overall system, however can be operated at relatively low peak pressures of 20 psi over ambient or less. The system of this invention accordingly overcomes the disadvantages of some suggested systems that require compressors on board to effect nitrogen separation from air.
Accordingly, in summary, the instant invention provides an on-board fuel inerting system for a vehicle subject to electrical sparking or other intrusion of potentially explosive occurrences within a fuel tank of the vehicle which comprises: (a) an oxygen partial pressure detector maintained in contact with the vapors in the ullage of the fuel tank; (b) a source of inert gas maintained on-board the vehicle in valved communication with the ullage volume of the fuel tank; and (c) a controller responsive to the detection of a partial pressure of oxygen within said ullage volume that is higher than some predetermined level to cause inert gas to flow into the ullage volume from said source to establish the oxygen partial pressure below said predetermined level. In the preferred embodiments, the system is adapted to an aircraft using nitrogen gas as the inerting agent and using a fiberoptic oxygen-detecting probe which makes it possible to detect oxygen partial pressure within the tank without exposing the ullage volume to electricity and an additional source of sparking.